Mixtures of waste materials containing desirable plastics and other materials are often processed to recover either or both of the plastic and other components. Such methods include burning the mixture to remove the plastic component, dissolving the plastic component in a chemical solvent to recover the non-plastic and a solution of the plastic component, electrostatic separation, separating the components by a flotation process, and separating by a thermal process.
U.S Pat. No. 3,268,071 discloses a process for separating a mixture of two comminuted solids. The mixture is suspended in a liquid, which is treated so as to cause one of the solids to be lyophilic and the other to be lyophobic A bridging liquid is thereafter added to cause the lyophobic particles to agglomerate for removal by, for example, filtering.
In those methods based upon a flotation process, density modifiers are often used to adjust the specific gravity of the bath, in order to float plastics having a greater specific gravity than the pure bath liquid U S. Pat. Nos. 4,578,184 and 4,728,045 generally disclose the use of a water bath which may be density-modified by the addition of salts to form an aqueous solution. These methods, however, are ineffective in separating plastics having similar specific gravities. Additionally, certain materials may not be separable by these methods due to particle size distribution effects, or small particle sizes which cause surface effects to dominate over gravitational forces.
A thermal process for separating polyvinyl chloride and polyethylene terephthalate is disclosed in "PVC Melt Point Keys Scrap Separation," Modern Plastics, June 1990. pp 15 and 16. A mixture of PVC and PET is precisely metered out onto a conveyor belt which transports it through a heating chamber maintained at the melt point for PVC. When the PVC becomes molten, it adheres to the conveyor belt. Separation occurs at the end of the conveyor belt, where the PET drops off and the PVC is scraped off of the belt.
It must be noted that the prior art referred to above has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention.
It would be desirable to develop a process for separating thermoplastics from mixtures containing the thermoplastic and a contaminant, which would provide a high level of purity of the thermoplastic material recovered, could be operated economically, and would not require post cleaning of the reclaimed thermoplastic prior to its reprocessing.